Polymer particle dispersions or emulsions are used in a number of applications, such as adhesives, paints, paper coatings, textile coatings, etc. Emulsion polymerization is a type of radical polymerization often used to form dispersions of polymer particles that usually begin with an emulsion incorporating water, at least one monomer, and surfactant(s). Surfactants are often critical in conventional emulsion polymerization because the surfactant provides sites for particle nucleation (i.e., monomer swollen micelles), controls latex particle size, and provides colloidal stability to the formed polymer particles. However, for many latex applications, small molecule surfactants have several disadvantages. For example, small molecule surfactants can impart high water sensitivity to films (leading to blush) and can desorb during film formation (detracting from surface properties and final performance).
There have been recent developments in providing effective emulsion polymerizations without the need for a surfactant, however. For example, an American Chemical Society manuscript entitled “Nitroxide-Mediated Controlled/Living Free-Radical Surfactant-Free Emulsion Polymerization of Methyl Methacrylate Using a Poly(methacrylic acid)-Based Macroalkyoxyamine Initiator” by Charleux et al. (Macromolecules 2009, 42, 95-103) describes the use of certain macroinitiators in a surfactant-free emulsion polymerization. In particular, a poly(methacrylic acid)-based polymer, such as poly(methacrylic acid-co-styrene) is described as a living macroinitiator used in a surfactant-free, batch emulsion polymerization.
It has been found, however, that emulsion polymerization using macroinitiators based on acrylic acid, methacrylic acid-co-styrene or methacrylic acid-co-styrene sulphonate sodium salt, for example, requires the latex to have a basic pH. This is because a low pH emulsion may lead to latex instability and coagulation, especially at higher solids contents, during synthesis or shortly thereafter.